Network operators and carriers are deploying frame-switched communications networks in place of circuit-switched networks. In frame-switched networks such as Internet Protocol (IP) networks, IP frames are forwarded according to routing state stored at each IP router in the network. Similarly, in Ethernet networks, Ethernet frames are forwarded according to forwarding state stored at each Ethernet switch in the network. The present invention applies to communications networks employing any Protocol Data Unit (PDU) based network and in this document, the terms “frame” and “frame-switched network”, “routing”, “frame” and “frame-based network”, “forwarding” and cognate terms are intended to cover any PDUs, communications networks using PDUs and the selective transmission of PDUs from network node to network node.
Multicast forwarding of data frames (where frames are sent from a source node to multiple destination nodes more or less simultaneously) is of increasing importance as demand for services such as Internet Protocol Television (IPTV) and Video on Demand (VoD) grows.
Protocols such as Intermediate System-Intermediate System (IS-IS) or Open Shortest Path First (OSPF) are used to disseminate network topology information used to calculate paths for forwarding frames from a plurality of source nodes to one or more destination nodes, typically through one or more intermediate nodes, and to install the forwarding state required to implement those paths. OSPF and IS-IS are run in a distributed manner across nodes of the network so that, for example, when a topology change occurs in the network such as a node or link failure, this information is flooded to all nodes by the protocol's operation, and each node will locally recompute paths to circumvent the failure based on a consistent view of network topology.
In Ethernet networks, Provider Backbone Transport (PBT), also known as Provider Back-Bone Bridging-Traffic Engineering (PBB-TE), as described in Applicant's British patent number GB 2422508 is used to provide a unicast Ethernet transport technology. Provider Link State Bridging (PLSB) as described in Applicant's co-pending U.S. patent application Ser. No. 11/537,775 will be used to provide a multicast transport capability for Ethernet networks using IS-IS to set up unicast paths and multicast trees in the network. Both above patent documents are hereby incorporated by reference.
Many network operators have deployed Multi Protocol Label Switching (MPLS) as their frame switched network transport technology, with an overlay technology called Virtual Private LAN Service (VPLS) providing the infrastructure for customer any-to-any connectivity (E-LAN services) delivered over restricted (typically metro scale) network domains. VPLS is an Ethernet LAN emulation provided over MPLS. Within this document, the terms VPLS and Ethernet LAN segment are used interchangeably to describe the service offered to the end-customer.
A problem with VPLS is that it scales poorly, in particular because customer Media Access Control (C-MAC) addresses are exposed to the VPLS domain. Further, VPLS constructs a full mesh of pseudo-wires between every node with a point of presence for any specific service, so the telemetry associated with a VPLS service instance scales in proportion to the square of the number of end points. Finally the full mesh of pseudo wires means that any flooding of frames is inefficient, as all frame replication must be performed at the ingress to the pseudo-wire mesh. In cases where the number of pseudo-wires exceeds the number of physical links traversed at a given point, multiple copies of the same frame will be sent on each physical link.
One approach to mitigating this scaling problem is to use Hierarchical VPLS (H-VPLS), which uses multiple hub and spoke architectures at the edges to contain the size of the fully-meshed transport core, and thus limit the number of transport connections required. This approach has the penalty alluded to above, in that the gateways between edge and core are exposed to the full range of C-MAC addresses, which was already a severe scaling limitation. It also introduces additional complexity to address resiliency issues as it requires multi-homing of the spokes onto the core mesh.
An increasingly preferred approach to the scaling problems of VPLS is to use Provider Backbone Bridges (PBBs)—standardized as IEEE 802.1ah—at the edges of the VPLS core, to separate the C-MAC address spaces from the operator backbone MAC (B-MAC) address space through encapsulation. In this way, a VPLS domain is typically exposed to a small number of B-MAC addresses summarizing a much larger set of C-MAC addresses which would typically be found on a customer LAN segment.
However the deployment of PBB overlaid on existing VPLS suffers limitations in that interworking between a PBB Network and legacy ports (i.e. where the peer VPLS Provider Edge router, PE, is not configured to support Backbone Edge Bridging) presents numerous challenges and complexity, and in any case the combination of VPLS and PBB only has the capability to address some of the scaling issues of VPLS.
An alternative approach is to migrate towards a PLSB core network as PLSB overcomes many of the shortcomings of VPLS with respect to multicast efficiency, resiliency and ease of provisioning. It is desirable to be able to do this without perturbing existing deployed customer facing VPLS ports and at the same time maximizing the utilization of deployed assets. Similarly where VPLS has been deployed in the core and the decision has been made to deploy PLSB in the metro, it is desirable to use the deployed MPLS/VPLS capacity until such point as network load and economics mandates direct interconnect of subtending PLSB metro area networks.
Therefore a means of resilient and efficient interconnect of PLSB and existing VPLS is highly desirable. This needs to be true where VPLS subtends the Link-State controlled domain (User Network Interface (UNI) interconnect) and where VPLS simply lends transit capacity to Link-State controlled domain (Network Network Interface (NNI) interconnect).